(1) Field of the Invention
The invention relates to a battery protection circuit, and more particularly, to a battery protection circuit having a method for measuring and filtering analog signals.
(2) Description of the Prior Art
Rechargeable batteries have gained wide acceptance and usage as power sources for portable appliances. Mobile communications devices, entertainment devices, and video devices are just a few of the many appliances now using rechargeable battery technology. An important feature of these devices is the circuitry that controls the charging and discharging of the battery. Over charging and over discharging of the battery cells can reduce the lifetime, or the performance or even destroy the battery.
To prevent over charging and over discharging, the battery control circuit must measure the charging current (CC), the charging voltage (CV), the discharging current (DC), and the discharging voltage (DV). The signals are analog in nature, and typically exhibit transient responses and noise. Therefore, it is necessary for the control circuit to measure and to filter these current and voltage signals to obtain values that can be used in the control circuit for deciding when to open and close the controlling switches.
In the current art, there are two techniques for measurement and filtering. In the first technique, a timebase is derived external to the control integrated circuit using a resistor-capacitor (RC) charge-discharge time constant circuit. This approach has the advantage a relatively low current consumption. Unfortunately, the external components are expensive to manufacture and assemble. The second technique uses an internally generated timebase to reduce cost. However, this internal timebase uses a considerable amount of power. It would be advantageous to perform the measurement and filtering function using an internal timebase with significantly lowered power consumption.
Several prior art inventions describe battery protection circuits. U.S. Pat. No. 5,764,033 to Tamai discloses a battery protection circuit to prevent over charging. U.S. Pat. No. 5,982,149 to Shih describes a mobile phone battery charger. U.S. Pat. No. 5,894,212 to Balogh teaches a discharge protection system for a multi-cell battery. A microprocessor is used to control sampling of cell voltages at a fast sample rate. U.S. Pat. No. 5,539,299 to Fernandez et al describes a protection switch for a battery pack.